Earnings per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended												12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Earnings Per Share, Basic and Diluted [Abstract]
Basic earnings per common share (in dollars per share)	$ 0.50	$ 0.49	$ 0.49	$ 0.43	$ 0.48	$ 0.44	$ 0.57	$ 0.40	$ 0.45	$ 0.48	$ 0.40	$ 0.36	$ 1.91	$ 1.89	$ 1.69
Diluted earnings per common share (in dollars per share)	$ 0.48	$ 0.46	$ 0.47	$ 0.40	$ 0.44	$ 0.41	$ 0.53	$ 0.37	$ 0.42	$ 0.43	$ 0.36	$ 0.32	$ 1.80	$ 1.75	$ 1.51
Basic weighted average shares outstanding (in shares)													36,948,000	37,742,000	34,845,000
Diluted weighted average shares outstanding (in shares)													39,100,000	40,889,000	39,018,000
Earnings Per Share, Diluted [Abstract]
Net income	$ 18,745	$ 18,072	$ 18,067	$ 15,685	$ 17,330	$ 16,536	$ 22,348	$ 15,164	$ 15,944	$ 16,681	$ 14,010	$ 12,384	$ 70,569	$ 71,378	$ 59,019
Convertible debt interest (excludes imputed interest)													0	0	10
Net income for diluted earnings per share purposes													$ 70,569	$ 71,378	$ 59,029
Diluted shares outstanding (in shares)													39,100,000	40,889,000	39,018,000
Diluted earnings per common share (in dollars per share)	$ 0.48	$ 0.46	$ 0.47	$ 0.40	$ 0.44	$ 0.41	$ 0.53	$ 0.37	$ 0.42	$ 0.43	$ 0.36	$ 0.32	$ 1.80	$ 1.75	$ 1.51
Antidilutive securities excluded from computation of earnings per share (in shares)													90,000	90,000	0
Weighted Average Number of Shares Outstanding, Diluted [Abstract]
Basic weighted average shares outstanding (in shares)													36,948,000	37,742,000	34,845,000
Incremental shares for common stock equivalents (in shares)													2,152,000	3,147,000	4,173,000
Diluted shares outstanding (in shares)													39,100,000	40,889,000	39,018,000